JP2730465B2 - Light control device and manufacturing method thereof - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION LiNb having electro-optic effect
Optical waveguide formed in O ₃ or LiTaO ₃ substrate
CONTROL DEVICE PERFORMING CONTROL USING CONTROL METHOD AND ITS MANUFACTURING METHOD

[0002]

2. Description of the Related Art With the practical use of optical communication systems, higher capacity and more sophisticated systems have been demanded, and new functions such as higher-speed generation of optical signals and switching and switching of optical transmission paths have been demanded. Is required. In the current practical optical communication system, an optical signal is obtained by directly modulating the injection current of a semiconductor laser or a light emitting diode. However, in direct modulation, a frequency of several GHz or more is used due to the effect of relaxation oscillation and wavelength chirping. Are difficult to apply to the coherent optical transmission system due to the difficulty in high-speed modulation of the optical transmission and wavelength fluctuation. As a means to solve this, there is a method using an external modulator. In particular, a waveguide type optical modulator composed of an optical waveguide formed in an electro-optic crystal substrate is characterized by its small size, high efficiency, and high speed. There is.

On the other hand, an optical switch is used as a means for obtaining a function of switching an optical transmission line and a function of switching a network. The optical switch currently in practical use is a prism,
It switches an optical path by mechanically moving a mirror, a fiber, and the like, and has disadvantages such as low speed, large shape, and unsuitability for matrix formation. As a means for solving this problem, a waveguide type optical switch using an optical waveguide is being developed, and has features such as high speed, integration of many elements, and high reliability. In particular, a device using a ferroelectric material such as lithium niobate (LiNbO ₃ ) has features such as low light absorption and low loss, and high efficiency due to a large electro-optic effect. Various types of optical control devices such as a directional coupler type optical modulator or an optical switch, a total reflection type optical switch, and a Mach-Zehnder type optical modulator have been reported.

In recent years, research and development on high-density integration of this waveguide type optical switch has been actively carried out. Nishimoto et al.
An 8 × 8 matrix optical switch in which 64 directional coupler optical switches are integrated using _three substrates has been reported. .
On the other hand, research and development of devices including a single optical switch element such as an external optical modulator are also actively pursued. The characteristics of such an optical switch device include stability of operation with respect to the environment such as switching voltage (power), crosstalk, extinction ratio, loss, switching speed, temperature and humidity, and stability of operation when voltage is continuously applied. There is sex.

[0005] Among the characteristic items described above, stable operation is the most important point in practical use. Here, the prior art will be described with reference to the drawings. FIG. 2 is a cross-sectional view showing a final form of a conventional optical switch using a directional coupler composed of optical waveguides 2a and 2b.

In FIG. 2, Li having an electro-optical effect is used.
A buffer layer 4 is loaded on a substrate 1 including a directional coupler composed of two optical waveguides 2a and 2b formed on an NbO ₃ or LiTaO ₃ substrate 1 (hereinafter referred to as a substrate 1). The electrodes 6a, 6b mainly made of a metal material are formed on the optical waveguides 2a, 2b through the intermediary.
Then, the conductive film 7 is further loaded. This conductive film 7 is made by Sawaki et al., CLEO '86 MF-2,4.
According to the paper on page 6, there is an effect of preventing the characteristic instability due to the movement of electric charges generated in the substrate due to the pyroelectric effect of the ferroelectric substance when the temperature fluctuates. That is, it is considered that there is an effect of stabilizing the switch operation with respect to temperature fluctuation, and a Si film is used.

The buffer layer 4 is composed of the optical waveguides 2a, 2b
Is used to prevent the light propagating through the substrate from being absorbed by the electrodes 6a and 6b and the conductive film 7, and an insulator which absorbs very little light, especially SiO ₂ or Al ₂ O ₃ is generally used. This is because these refractive indexes are smaller than the refractive index of the LiNbO ₃ or LiTaO ₃ substrate 1 having the electro-optical effect of about 2.2 and there is almost no light absorption. When the refractive index is small, the electrodes 6a, 6b
In addition, the thickness of the buffer layer 4 necessary for preventing light absorption by the conductive film 7 can be made thinner than when the refractive index is large.
Considering the switching voltage, when a voltage is applied to the electrodes 6a and 6b, the electric field concentrates on the buffer layer 4 because the dielectric constant of the buffer layer 4 is usually smaller than that of the substrate 1, and the switching is performed as the thickness of the buffer layer 4 increases. The voltage increases. Therefore, SiO ₂ having a small refractive index and extremely small light absorption is used as the buffer layer 4.

[0008]

In such conventional optical switching, when the buffer layer 4 is made of SiO ₂ and the electrode 6a is grounded and a DC voltage is continuously applied to the electrode 6b, ions in the buffer layer 4 are converted into electric fields. And the ions having the opposite sign to the sign of the voltage applied from the outside gather under each of the electrodes 6a and 6b. Therefore, a counter-electric field is generated between the electrodes 6a and 6b with respect to the electric field in the substrate generated by an externally applied voltage. The magnitude of this anti-electric field increases with the increase in the total movement amount of ions with time. This phenomenon is generally called DC drift. When the voltage applied from the outside is constant, when an anti-electric field is generated, the electric field applied to the optical waveguide decreases, and the characteristics are deteriorated. That is, when an anti-electric field that increases with time is generated, a voltage for canceling the anti-electric field must be applied from the outside in order to return to the characteristics before the generation of the anti-electric field. This means a shift of the operating voltage point of the switch operation, which is a major obstacle for practical use. Further, these ions are mixed into the buffer layer 4 as impurities in the manufacturing process of the buffer layer 4. Among the impurity ions, alkali metals such as Na, Li and K are known to be easily ionized and move easily in the buffer layer 4 as mobile ions, and it is necessary to prevent the invasion of these ions. But,
In LiNbO ₃ or LiTaO ₃ , when heat treatment is performed, an external diffusion phenomenon occurs in which Li ₂ O is released from the crystal surface to the outside. Therefore, when heat treatment is performed particularly when the buffer layer 4 is formed on the surface of the substrate 1, The Li ₂ O is mixed into the buffer layer 4 due to the external diffusion and becomes Li ions. When the electric field is applied after the device is manufactured, the buffer layer 4 moves in the buffer layer 4 to form an anti-electric field, which causes a DC drift.

Further, in the structure in which the buffer layer is removed, it is difficult to design impedance matching of a circuit in a device requiring high frequency characteristics such as an optical modulator. Doing so will limit the application range of the device.

An object of the present invention is to provide a light control device capable of obtaining a stable operation for a long time as compared with a conventional light control device.

[0011]

According to the present invention, there is provided a method for manufacturing an optical control device, an optical waveguide formed on a LiNbO ₃ or LiTaO ₃ substrate having an electro-optical effect, a buffer layer formed on at least the optical waveguide, In the method of manufacturing a waveguide-type light control device including an electrode formed on the buffer layer near the optical waveguide, a metal film or a semiconductor film is formed on the substrate so that the thickness is larger than other regions on the optical waveguide. After the formation, a buffer layer is formed by changing the metal film or the semiconductor film into an insulator film by oxidation or the like while leaving a part on the optical waveguide. The light control device of the present invention further comprises a LiNbO ₃ or LiTaO ₃ substrate having an electro-optical effect, an optical waveguide formed on the substrate, an insulating buffer layer formed at least on the optical waveguide, In a waveguide type light control device including an electrode formed on the buffer layer in the vicinity, a metal film or a semiconductor film is provided only on the surface where the optical waveguide is formed between the optical waveguide and the buffer layer. It is characterized by the following. Further, the light control device of the present invention has LiNbO ₃ having an electro-optical effect.
Alternatively, it comprises a LiTaO ₃ substrate, a plurality of optical waveguides formed on the substrate, at least an insulating buffer layer formed on the optical waveguide, and an electrode formed on the buffer layer near the optical waveguide. In the optical control device of the waveguide type, the buffer layer is separated for each optical waveguide, and has a metal film or a semiconductor film only on the surface where the optical waveguide between the optical waveguide and the buffer layer is formed. And Further, the method for manufacturing the light control device includes LiNbO ₃ or LiNb having an electro-optical effect.
A method for manufacturing a waveguide-type light control device, comprising: an optical waveguide formed on a TaO ₃ substrate; an insulating buffer layer formed on the optical waveguide; and an electrode formed on the buffer layer near the optical waveguide. 3. The method for manufacturing an optical control device according to claim 1, wherein a buffer layer is formed by depositing a semiconductor film on the substrate having the optical waveguide and then changing the semiconductor film to an insulator film by oxidation or the like. Further, a conductive film is provided on the electrode.

[0012]

In the method of manufacturing a light control device according to the present invention, first, as shown in FIG. 1A, a metal film or a semiconductor film 3 is deposited on a substrate 1 including a waveguide as a substitute for a buffer layer. I do. The metal or semiconductor film prevents diffusion of impurity ions from the substrate in a process such as thermal oxidation. In addition, since there is a substitute for the buffer layer, by controlling this buffer layer, the dielectric constant of the substrate can be changed, and impedance matching and low voltage of a high-bandwidth optical device can be achieved.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, the present invention will be described with reference to the drawings. 1A to 1D are cross-sectional views showing steps of a method for manufacturing a light control device according to one embodiment of the present invention, and the same parts as those in FIG. 2 are denoted by the same reference numerals.

FIG. 1A shows Li having an electro-optical effect.
In a method of manufacturing a directional coupler type optical switch including two optical waveguides 2a and 2b formed on an NbO ₃ or LiTaO ₃ substrate 1, a semiconductor or metal is formed on a substrate 1 including two optical waveguides 2a and 2b. FIG. 4 is a cross-sectional view in a state where a membrane 3 is loaded. The material used for the semiconductor or metal film 3 is Si, Ti, which has a low ionic conductivity and can be changed to a high-resistance insulator film by a process such as oxidation.
It is effective to use a material such as Ni, Cu, V, Co, Zn, which changes into an insulator by thermal oxidation or the like and can control the dielectric constant of the film.
In the manufacturing method according to the present invention, first, as shown in FIG. 1B, the semiconductor or metal film 3 other than on the waveguide is etched in advance, and then the semiconductor or metal film 3 is heat-treated in an oxygen atmosphere. The film is changed to the high-resistance insulator film 5 in the depth direction. The amount of change at this time is equal to the amount of change when the entire metal film portion from the surface of the metal film 3 appearing by etching to the surface of the substrate 1 is changed to a high-resistance insulator film. As shown in (c), it is desirable that the high resistance insulator film 5 separates the metal films 3a and 3b from each other. Finally, as shown in FIG.
After providing b, a conductive film 7 which is an antistatic film is formed.

As shown in FIG. 1D, the optical control device according to the manufacturing method described above is loaded on the optical waveguides 2a and 2b via the metal films 3a and 3b and the high-resistance dielectric film 5. Even when a voltage is applied by the electrodes 6a and 6b, the metal films 3a and 3b prevent the outward diffusion of Li ions from the substrate 1 to the buffer layer, thereby preventing the penetration of Li ions into the high-resistance dielectric film 5. No anti-electric field is generated by the movement of ions. Therefore, it is possible to obtain an optical control device capable of performing a stable operation for a long period without DC drift.

FIG. 3 shows another embodiment. FIG.
The difference from (d) is that the high-resistance dielectric film 5 is separated. In FIG. 1D, diffusion of impurity ions from the substrate 1 toward the electrode is prevented, but there is no measure against ion migration caused in the high-resistance dielectric film 5. In the embodiment of FIG. 3, the high-resistance dielectric film is 5a,
It is separated by an etching process as shown in FIG. Thereby, ion migration in the high-resistance dielectric film can be suppressed, and the reliability of the device can be further improved.

Further, when the propagation light loss is more problematic than the diffusion of impurity ions, the semiconductor or metal film is oxidized by a process such as oxidation as shown in the embodiment of FIG.
It can be applied by changing all of them to high-resistance insulator films.

The above process is effective for all light control devices such as a directional coupler type, a Mach-Zehnder type, and a total reflection type using a cross waveguide in a thermal diffusion waveguide device.

[0019]

As described above, according to the optical control device manufacturing method of the present invention, by inserting a metal film between the high-resistance dielectric film and the substrate, the impurity ions can be transferred from the substrate to the buffer layer. Since the contamination can be prevented, the migration and polarization of impurity ions in the high-resistance dielectric film cause D.
There is an effect that a light control device with stable operation can be obtained without C drift. Further, the buffer layer facilitates device design such as device matching such as impedance matching, and lowers the voltage of the device.

[Brief description of the drawings]

FIGS. 1A to 1D are cross-sectional views illustrating a process for manufacturing a light control device according to an embodiment of the present invention.

FIG. 2 is a sectional view of a final form of a conventional light control device.

FIG. 3 is a sectional view of a final form of a light control device according to another embodiment of the present invention.

FIG. 4 is a sectional view of a final form of a light control device according to still another embodiment of the present invention.

[Explanation of symbols]

1 LiNbO ₃ or LiTaO ₃ substrate 2a, 2b optical waveguide 3, 3a, 3b metal film 4 buffer layer 5 high resistance insulation film 6a, 6b electrodes 7 conductive film

 ──────────────────────────────────────────────────続 き Continuation of front page (72) Inventor Naoki Kitamura 5-7-1 Shiba, Minato-ku, Tokyo NEC Corporation (56) References JP-A-7-20508 (JP, A) JP-A-7 -64126 (JP, A) JP-A-60-86529 (JP, A)

Claims (6)

(57) [Claims] An optical waveguide formed on a LiNbO ₃ or LiTaO ₃ substrate having an electro-optic effect, at least a buffer layer formed on the optical waveguide, and an electrode formed on the buffer layer near the optical waveguide. In the method for manufacturing a waveguide-type light control device, after forming a metal film or a semiconductor film on the substrate so as to be thicker than other regions on the optical waveguide, the metal film or the semiconductor film is oxidized or the like. A method for manufacturing a light control device, comprising forming a buffer layer by changing a part of the optical waveguide into an insulator film while leaving a part on the optical waveguide. 2. A LiNbO ₃ or LiTaO ₃ substrate having an electro-optic effect, an optical waveguide formed on the substrate, an insulator buffer layer formed at least on the optical waveguide, and the buffer near the optical waveguide. In a waveguide-type light control device including electrodes formed on a layer, a metal film or a semiconductor film is provided only on the surface where the optical waveguide between the optical waveguide and the buffer layer is formed. Light control device. 3. A LiNbO ₃ or LiTaO ₃ substrate having an electro-optic effect, a plurality of optical waveguides formed on the substrate, an insulator buffer layer formed at least on the optical waveguide, In a waveguide-type light control device including an electrode formed on the buffer layer, the buffer layer is separated for each optical waveguide, and on a surface on which the optical waveguide is formed between the optical waveguide and the buffer layer. An optical control device comprising a metal film or a semiconductor film only in the light control device. 4. The electro-optical effect LiNbO ₃ or LiTaO ₃ optical waveguide formed on a substrate having a, the optical waveguide on the formed buffer layer, from electrode formed on the buffer layer of the optical waveguide near In the method for manufacturing a waveguide-type light control device, a semiconductor film is deposited on a substrate having the optical waveguide, and then the buffer layer is formed by changing the semiconductor film to an insulator film by oxidation or the like. A method for manufacturing a light control device. 5. A light control device according to claim 2, wherein a conductive film on the electrode. 6. The method for producing a light control device according to claim 1, wherein the forming a conductive layer on the electrode.